1. Field of the Invention
This invention relates to magnetic heads for writing to and/or reading from magnetic information storage media, particularly such heads formed by thin film techniques. The invention is particularly though not exclusively applicable to such heads for use in magnetic hard disc storage devices. The invention also relates to a method of making such a head.
2. Description of the Prior Art
The storage capacity required of magnetic hard disc storage devices has become greater and greater, with the increase in the processing capacity of computers. Consequently, since it is desired not to increase the size of such hard disc devices, it is sought to reduce the track width of the disc, which means also reducing the track width of the magnetic pole pieces of magnetic heads used for writing to and reading from such discs. It is also necessary to control the pole thickness (pole length) of these magnetic heads. When manufacturing such magnetic heads, it is important to achieve high accuracy in both the track width of the head and the pole thickness, in order to avoid an unacceptably high number of low quality or defective products.
Among many factors relevant to the bit length on the disc, pole thickness of the magnetic head is significant.
Thin film techniques have been applied to the production of magnetic heads having very small track width and pole thickness. These techniques involve forming a number of layers on a substrate. The substrate in this case is typically a ceramic slider of the magnetic head.
The structure of a prior art thin film magnetic head will be explained with reference to FIGS. 2(a) and 2(b) which are perspective and sectional views showing the structure of the principal portions of the head.
A bottom magnetic film 22 is formed on a substrate 21 and constitutes a bottom magnetic pole piece forming a magnetic circuit in cooperation with an top magnetic pole piece film 23 which is to be formed later. A non-magnetic material 25 is sandwiched between the top and bottom magnetic films 23 and 22 at a tip portion 24, forming a magnetic gap. Write and read operations are conducted by the top and bottom magnetic films 23, 22 and the magnetic gap.
At the central portion of the magnetic head, a conductor coil 26 is disposed between the pole pieces so as to encircle and cross the magnetic circuit. This conductor coil 26 is insulated from the top and bottom magnetic films 23, 22 by insulator films 27. The upper part of the device is covered with a thick protection film 28.
The recording density of the magnetic disc apparatus using this thin film magnetic head is determined primarily by the shape of the tip portion 24. Particularly, the width d of the top magnetic film 23 which determines the track width and a pole thickness t (the sum of the thickness of the bottom magnetic film 22, the magnetic gap and top magnetic film 23) which determines over-write and resolution must be formed very accurately. To form the value d highly accurately, the pattern of the top magnetic film 23 must be formed highly accurately at the lower part of the step down from the insulator films 27 which has a height of about 10 .mu.m. However, when a photosensitive resin is coated on such a high step, the photosensitive resin at the region next to the step is as thick as about at least 10 .mu.m so that a highly accurate pattern of the film 23 cannot be formed.
Accordingly, it has been proposed to form the tip portion of the top pole piece separately from a rear portion thereof.
In Japanese Laid-Open Patent Application No. JP-A-60-10409 for example, only the tip portion of the top magnetic film is formed before the insulator film is formed and thereafter the insulator film and the rear portion of the top magnetic film are formed. According to this method, the width of the top magnetic film can be determined when the step hardly exists, and this document describes that the track width can be made highly accurately.
Japanese Laid-Open Patent Applications Nos. JP-A-62-62415 and JP-A-63-168811 disclose methods in which the tip portion of the top magnetic film is formed after the lowermost layer of the insulator is formed, and then the upper layer of the insulator and the rear portion of the top magnetic film are formed. By these methods also, the width of the top magnetic film can be determined when the step is low, and the documents describe that the track width can be made highly accurately.
A method in which each of the top and bottom magnetic films is formed by dividing it into a tip portion and a rear portion is disclosed in Japanese Laid-Open Patent Application No. JP-A-60-10410. According to this method, it is possible to form the tip portion of the magnetic film of a high saturation magnetization alloy and the rear portion of a high permeability magnetic material. The document states that a thin film magnetic head having excellent recording characteristics can be obtained.
While the first three of these prior art documents describe formation of the tip portion of the top pole piece, they have failed to consider the effect of subsequent processing on the tip portion. The etching of the layer for the rear portion of the top pole piece will affect the tip portion as well, both in width and thickness. This problem and its effect on the important factor of pole thickness has not been considered.
The influence exerted by the fluctuation of the pole thickness or length on the characteristics of the thin film magnetic head will be explained with reference to FIG. 6. FIG. 6 shows the relation between the over-write value and the resolution value representing the write capacity and read capacity of the thin film magnetic head, respectively, and the pole length. This diagram is disclosed in M. Hanazono et al., "Design and Fabrication of Thin-Film Heads Based on a Dry Process", Journal of Applied Physics, Vol. 61, No. 8, Part IIB, Apr. 15, 1987, pp. 4157-4162.
As shown in FIG. 6, when the pole length increases, over-write increases but resolution drops. In a thin film magnetic head, both over-write and resolution are preferably large so that the pole length must be controlled strictly to a value within a certain range. More definitely, it has been found that unless the thin film magnetic head is produced while controlling the pole length within about .+-.10% of a target value, good characteristics cannot be produced stably.
However, when any of the prior art techniques described above are applied, the pole thickness or length fluctuates because after the tip, portion of the top magnetic film is formed, the insulator layer, the conductor coil and the rear portion of the top magnetic film are laminated. During this, the surface of the tip portion of the top magnetic film that has been formed previously is etched or damaged.
Generally, the conductor coil is formed by ion milling or electro-plating, for example, and the surface of the tip portion of the top magnetic film is unavoidably etched by over-etching at the time of ion milling or during the etching step of a plating base film. Similarly, the surface of the tip portion of the top magnetic film is unavoidably etched during the formation step of the rear portion of the top magnetic film and hence the fluctuation of the pole length is unavoidable. It might be possible to form, in advance, the tip portion of the top magnetic film with greater thickness, so as to compensate for the decrease of the pole length by this etching and to attain a target pole length after etching. In such a case, too, variance of etching at the time of formation of the conductor coil and the rear portion of the top magnetic film adds to variance of the film thicknesses of the bottom magnetic film, magnetic gap film and tip portion of the top magnetic film. For this reason, the fluctuation of the pole length is so great that it cannot be controlled within the range of .+-.10% described above.
In JP-A-60-10410 cited above, there is disclosed a method which divides the top magnetic film into two parts and forms the tip portion after the rear portion is formed. If this method is employed, the surface of the tip portion of the top magnetic film is not etched during the production process, but the pattern of the tip portion which determines the track width must be formed at the lower part of the step having a height of about 10 .mu.m. Thus, there is left the problem that the track width cannot be made highly accurate.
When the relation between the structure of a practical thin film magnetic head and its recording characteristics is taken into consideration, the relation of position of the connection between the tip portion and the rear portion of the top magnetic film is extremely important, and exerts a significant influence on the recording characteristics of the head. However, none of the prior art disclosures described above consider the shapes and positions of the tip portion and rear portion of the top magnetic film.